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arxiv: 2201.05205 · v2 · submitted 2022-01-13 · ✦ hep-ex

Azimuthal Asymmetries in Unpolarised Semi-Inclusive DIS at COMPASS

Jan Matousek (on behalf of the COMPASS Collaboration) This is my paper

Pith reviewed 2026-05-24 12:00 UTC · model grok-4.3

classification ✦ hep-ex
keywords semi-inclusive deep inelastic scatteringazimuthal asymmetriesCOMPASS experimentBoer-Mulders functionCahn effecttransverse momentum dependent PDFsvector meson background
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The pith

COMPASS extracts amplitudes of three azimuthal modulations in unpolarised SIDIS on a hydrogen target after new vector-meson background subtraction.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

In semi-inclusive deep inelastic scattering the transverse momentum of partons produces azimuthal dependence in the cross section even when the target nucleon is unpolarised. Three modulations appear: the cos φ_h term from the Cahn effect, the cos 2φ_h term tied to the Boer-Mulders distribution, and the sin φ_h term that is a twist-three beam-spin asymmetry. The 2016-2017 COMPASS run used a 160 GeV muon beam on liquid hydrogen and extracted the amplitudes of these modulations from part of the data set. A new subtraction removes events from the decay of diffractively produced vector mesons, and the resulting amplitudes match earlier COMPASS measurements taken on an isoscalar target.

Core claim

Amplitudes of the cos φ_h, cos 2φ_h and sin φ_h azimuthal modulations have been extracted from 2016-2017 COMPASS data collected with a longitudinally polarised 160 GeV/c muon beam scattering on a liquid hydrogen target; a newly developed procedure subtracts background from decays of diffractively produced vector mesons, and the extracted amplitudes qualitatively agree with previous COMPASS results obtained on an isoscalar target.

What carries the argument

The new procedure to subtract background coming from the decay of diffractively produced vector mesons, which isolates the semi-inclusive deep inelastic scattering sample used to extract the three azimuthal amplitudes.

If this is right

  • The extracted amplitudes constrain combinations of transverse-momentum-dependent parton distributions and fragmentation functions at the measured kinematics.
  • Qualitative agreement between the hydrogen-target results and earlier isoscalar-target results indicates that the modulations are similar for proton and neutron.
  • The subtraction method can be applied to the remaining 2016-2017 data to increase the precision of the amplitude measurements.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the subtraction is unbiased it could be adapted for other fixed-target SIDIS experiments that face similar diffractive backgrounds.
  • The observed agreement between targets suggests that nuclear corrections to these particular azimuthal modulations are small at the COMPASS kinematics.
  • Higher-statistics samples could allow the cos 2φ_h amplitude to be separated into contributions from different quark flavours.

Load-bearing premise

The new background-subtraction procedure removes contamination from vector-meson decays without introducing bias into the extracted azimuthal amplitudes.

What would settle it

A statistically significant difference between the azimuthal amplitudes obtained before and after the new subtraction, when both are applied to the same data set in a control region dominated by vector-meson decays.

Figures

Figures reproduced from arXiv: 2201.05205 by Jan Matousek (on behalf of the COMPASS Collaboration).

Figure 1
Figure 1. Figure 1: The momentum q of the γ ∗ defines the z axis of the γ ∗N system. Ph is the momentum of the observed hadron h [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 3
Figure 3. Figure 3: The amplitudes of the cos ϕh modulation in bins of x, z and PT in the full Q 2 range (top row) and divided in four Q 2 ranges. 1 10 [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: The amplitudes of the cos 2ϕh modulation in bins of x, z and PT in the full Q 2 range (top row) and divided in four Q 2 ranges. 1 10 [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: The amplitudes of the sin ϕh modulation in bins of x, z and PT 5 [PITH_FULL_IMAGE:figures/full_fig_p005_7.png] view at source ↗
read the original abstract

In semi-inclusive deep inelastic scattering (SIDIS) the non-zero transverse momentum of partons induces azimuthal dependence of the cross-section. For unpolarised nucleon, three azimuthal modulations that can be related to different combinations of twist-two or higher twist transverse momentum dependent PDFs and fragmentation functions arise: the so-called Cahn effect reflected in $\cos\phi_\mathrm{h}$ modulation, the $\cos2\phi_\mathrm{h}$ term related to the Boer--Mulders PDF and $\sin\phi_\mathrm{h}$ twist-three effect known as beam-spin asymmetry. In 2016 and 2017, the COMPASS experiment at CERN collected a large sample of SIDIS events using a longitudinally polarised 160~GeV/$c$ muon beam scattering on a liquid hydrogen target. Amplitudes of the aforementioned azimuthal modulations have been extracted from part of the data. A new procedure has been developed to subtract a background coming from the decay of diffractively produced vector mesons. The results presented in this talk qualitatively agree with earlier COMPASS results obtained with an isoscalar target.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript reports extraction of azimuthal modulation amplitudes (Cahn cos phi_h, Boer-Mulders cos 2 phi_h, and beam-spin sin phi_h) in unpolarised SIDIS from a subset of the 2016-2017 COMPASS data taken with a 160 GeV/c muon beam on a liquid hydrogen target. A new procedure is introduced to subtract background events from decays of diffractively produced vector mesons, after which the amplitudes are stated to agree qualitatively with prior COMPASS results on an isoscalar target.

Significance. If the background subtraction can be shown to be unbiased, the measurement would supply proton-target data on these TMD-related asymmetries, extending the existing deuteron-target results and aiding global fits of transverse-momentum-dependent distributions. The development of a dedicated subtraction method for diffractive vector-meson contamination is a constructive technical contribution.

major comments (1)
  1. [Abstract] Abstract (paragraph on 2016-2017 analysis): the new background-subtraction procedure is asserted to remove contamination from diffractive vector-meson decays, yet the text supplies no description of the background model (kinematic distributions, Monte Carlo generator, or data-driven template), the normalization or subtraction algorithm (global or bin-by-bin), any closure tests on simulated samples, or the residual systematic uncertainty assigned to the three extracted amplitudes. Because the central claim is that reliable amplitudes have been obtained after this subtraction, the absence of these elements prevents assessment of possible bias.
minor comments (2)
  1. [Abstract] The fraction of the 2016-2017 data sample actually analysed is not stated.
  2. [Abstract] No kinematic selection cuts (Q^2, x_Bj, z, p_T^h) or event yields are provided.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful review and the positive assessment of the work's significance. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract (paragraph on 2016-2017 analysis): the new background-subtraction procedure is asserted to remove contamination from diffractive vector-meson decays, yet the text supplies no description of the background model (kinematic distributions, Monte Carlo generator, or data-driven template), the normalization or subtraction algorithm (global or bin-by-bin), any closure tests on simulated samples, or the residual systematic uncertainty assigned to the three extracted amplitudes. Because the central claim is that reliable amplitudes have been obtained after this subtraction, the absence of these elements prevents assessment of possible bias.

    Authors: We agree that the abstract, as a concise summary, does not contain the technical details requested. The full manuscript body includes a dedicated description of the background model (using a Monte Carlo generator for diffractive vector-meson production), the bin-by-bin subtraction algorithm, closure tests on simulated samples, and the assigned residual systematic uncertainties on the amplitudes. To make this immediately clear to readers, we will expand the abstract with a brief reference to the validated subtraction procedure and its systematic treatment. revision: yes

Circularity Check

0 steps flagged

No circularity: direct experimental measurement with no derivation chain

full rationale

This is an experimental report on SIDIS azimuthal asymmetry extraction from 2016-2017 COMPASS LH2 data. The text describes data collection, a new background subtraction procedure for diffractive vector-meson decays, and amplitude extraction for cos φ_h, cos 2φ_h, and sin φ_h modulations. No equations, model fits, or first-principles derivations are present that could reduce to inputs by construction. Results are stated to qualitatively agree with prior measurements, but this is an external comparison, not a self-referential loop. The analysis is self-contained against external data benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The paper is an experimental measurement report; its central claim rests on the correctness of event selection, the new vector-meson background subtraction, and the standard interpretation of azimuthal modulations in terms of TMD PDFs and fragmentation functions. No free parameters or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption Azimuthal modulations in unpolarised SIDIS arise from transverse-momentum-dependent PDFs and fragmentation functions (Cahn, Boer-Mulders, twist-three beam-spin terms)
    Invoked in the first sentence of the abstract as the physical basis for the three modulations studied.

pith-pipeline@v0.9.0 · 5723 in / 1334 out tokens · 30022 ms · 2026-05-24T12:00:57.483510+00:00 · methodology

discussion (0)

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Reference graph

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